The development of computed tomography (CT) technologies first pioneered by Hounsfield and Cormack were an important breakthrough in the field of radiology. CT scanners are now widely used for diagnostic medical imaging and industrial and security inspection applications. Micro-computed tomography (micro-CT) has recently emerged as a promising non-invasive imaging tool for biomedical research. It has been applied to the high-resolution imaging of bone structures and soft tissues (with the aid of contrast agents) of small animals. The typical design of a micro-CT scanner differs from other CT scanners in that typically the object, rather than the x-ray source, is rotated to collect the projection images for reconstruction. Cone-beam geometry and 2D x-ray detectors are commonly used such that the entire object can be directly reconstructed from a set of recorded 2D images.
The spatial resolution of the micro-CT scanner depends primarily on the x-ray focal spot size, the resolution of the detector, and the scanner geometry. The temporal resolution is determined by the x-ray exposure time and data collection speed of the detector. Although there have been significant innovations and improvements in the x-ray detection technology and imaging algorithms, the basic mechanism of generating x-ray radiation has remained the same. The limitations of the current micro-CT scanners, and to a large extent other CT systems, result primarily to the limitations of their x-ray sources.
Commercial x-ray sources typically use thermionic cathodes to generate the electrons used to produce x-ray radiation. The thermal process used in such devices has several inherent limitations including high operating temperature, slow response time, and the production of electrons having a random spatial distribution. The high operating temperature can result in a short cathode lifetime due to breakage of the cathode filament, and x-ray tubes requiring a large size. To provide the small focal spot size required for high spatial resolution, complicated electron optics are employed. As a result, micro-focus x-ray tubes are typically bulky, costly and have limited lifetime.
In addition to requiring high operating temperatures, thermionic emission is inherently a relatively slow emission process. Conventional x-ray tubes rely on mechanical shutters to switch on and off the x-ray exposure, which can result in slow response times. Grid-controlled x-ray tubes have been developed that provide improved response time and short x-ray pulse width, but the temporal resolution of such tubes is still limited and the x-ray waveform can not be easily programmed. The low temporal resolution and the large number of projection images required for reconstruction have prevented dynamic CT imaging of moving objects such as hearts which are important for diagnosis of coronary artery disease.
Although “ultra-fast” CT scanners such as the Dynamic Spatial Reconstructor and the electron-beam CT (EBCT) scanner with scanning time of less than 100 msec have been developed for such purposes, these systems can be much larger that other CT systems, limiting their availability for use. Recent research has shown that it is also possible to obtain dynamic information using conventional CT with spiral capability and fast rotation speed with electrocardiograph (ECG) triggering. But dynamic cardiac CT imaging has not been demonstrated using micro-CT scanners.
Electron field emission is a quantum process where under a sufficiently high external electrical field electrons can escape from the metal surface to the vacuum level by tunneling. Electron field emission is preferred to thermionic emission, as heating is not required and the emission current can be controlled by the external field to give instantaneous response time. In addition field emitted electrons are confined to a narrow cone angle along the electrical field direction, whereas thermal electrons can be spatially randomly distributed. The basic physics of field emission is summarized by the Fowler-Nordheim equation,I=αV2exp(−bφ3/2/βV)  (1)which states that the emission current (I) increases exponentially with increasing voltage (V). For a metal with a flat surface, the threshold field required for electron emission is typically around 104V/μm, which is impractically high. Consequently, electron field emitters rely on field enhancement (β) at sharp tips or protrusions of the emitter. One way to fabricate sharp tipped field emitters is by a lithography process. Such emitters, called Spindt tip emitters, have not been used in practical devices because of low emission current, poor stability, and high cost.
X-ray tubes using field emission cathodes have been investigated in the past. In the early systems, metal tips were used as the cathodes. Electrons were extracted by applying a pulsed high voltage between the target and cathode using Max generators, which use a series of discharging capacitors to generate the required threshold field. X-ray radiation is generated when the field emitted electrons bombard on target. The advantages of field emission x-ray tubes as compared to thermionic x-ray tubes in terms of their resolution and required exposure time have been demonstrated in clinical studies. The metal-tip emitters of these early systems were shown to be inefficient. The x-ray tubes were shown to have a limited lifetime of about 200 to 300 exposures, and exhibited slow repetition rates. In addition, with the diode configuration of the tubes, the acceleration voltage and the tube current could not be independently controlled. Field emission x-ray tubes using other types of emitters, such as the Spindt tips described above and diamond emitters, have also been investigated. The highest electron current demonstrated in these x-ray tubes has only been on the order of micro amps.
The carbon nanotube (CNT) is a relatively new carbon allotrope discovered about a decade ago. A CNT includes either a single graphene shell, referred to as a single-walled carbon nanotube (SWNT), or multiple concentric graphene shells, referred to as multi-walled carbon nanotube (MWNT). CNTs are typically about 1–50 nm in diameter and 1–10 μm in length. Considerable progress has been made recently in the fabrication of CNTs with controlled structure and morphology. Technologies have been developed for assembly and integration of CNTs into device structures.
Research has shown that CNTs are promising electron field emitters. The atomically sharp tips and large aspect ratios (typically >103) of CNTs provide for large field enhancement factors (β), thus requiring lower threshold fields for emission than other types of emitters such as the Spindt tips. In addition, the field emitted electrons have been shown to have an energy spread of ˜0.5 eV and a spatial divergence angle in a direction parallel to the electrical field of less than 5° degree half angle. CNT emitters have been shown to be stable at high currents. For example, a stable emission current of >1 μA (>106 A/cm2 density) has been observed from an individual SWNT. Macroscopic cathodes have been demonstrated to emit stable emissions of over 200 mA from a 3 mm diameter sample under DC operating conditions, and a peak emission current of 3000 A from a 9 cm cathode at 1 μs pulse width at 200 KV anode voltage. These properties make the CNT emitters attractive for various device applications. For example, field emission flat panel displays (FEDs), lighting elements, and discharge tubes for over-voltage protection have been demonstrated having CNT-based “cold” cathode emitters.